1. Field of the Invention
The present invention relates to a semiconductor laser element, an integrated semiconductor laser element, and a method for producing a semiconductor laser element.
2. Description of the Related Art
Distributed reflector (DR) semiconductor laser elements are known as highly effective semiconductor lasers suitable for high-capacity transmission and long haul transmission (Japanese Laid-open Patent Publication No. H5-48214 (hereinafter to be referred to as Patent Literature) and SeungHun Lee et al., “High Optical Feedback-Tolerance of Distributed Reflector Lasers with Wire-like Active Regions for High Speed Isolator-Free Operation”, CLEO/IQEC 2009, CTuH5. (hereinafter to be referred to as Non-Patent Literature)). A distributed reflector semiconductor laser element is provided with first and second diffraction grating areas formed integrally with each other along a waveguide core layer oscillating and guiding a laser light, and reflectivities differ from each other. The waveguide core layer adjacent to the first diffraction grating area is an active layer which illuminates when a current is injected thereinto and functions as a distributed feedback laser, and the second diffraction grating area functions as a passive reflector (Bragg reflector) for a laser light produced by the distributed feedback laser with no current being injected the second diffraction grating area.
Herein the distributed feedback laser element outputs laser lights from both of end faces in the same powers. By contrast, the distributed reflector semiconductor laser element described in Patent Literature outputs a laser light from only an end face at the side of the distributed feedback laser since a laser light propagating from the distributed feedback laser to the passive reflector's side is reflected by the passive reflector. As a result, a differential quantum efficiency of a single end of the distributed reflector semiconductor laser element is supposed to be approximately doubled relative to that of the distributed feedback laser element.
In a case of adapting a configuration of a distributed reflector semiconductor laser in which a waveguide core layer is deleted from a region functioning as a passive reflector, a region functioning as a distributed feedback laser must be formed in a process which is other than a process of forming a region functioning as a passive reflector. Therefore, a production error in the thickness, the composition or the like of the semiconductor layer tends to produce a difference between propagation constants of the region functioning as the distributed feedback laser and the region functioning as the passive reflector, thus causing a poor production yield.
As a means for solving the problem, Patent Literature discloses a configuration in which no waveguide core layer is deleted from the second diffraction grating area functioning as the passive reflector and in which values of coupling coefficients κ are different by differentiating depths of grating grooves of diffraction gratings between the first diffraction grating area and the second diffraction grating area. Since this configuration solves the problem of difference between propagation constants, no processes are required for removing a part of an active layer or matching propagation constants, thus a distributed reflector semiconductor laser can be achieved that is superior in productivity.
In a distributed reflector semiconductor laser element described in Non-Patent Literature, any waveguide core layers are disposed discretely at a predetermined period in both an area functioning as a distributed feedback laser and an area functioning as a passive reflector to form diffraction gratings. The distributed reflector semiconductor laser element described in Non-Patent Literature is supposed to be capable of increasing, by the above-described configuration, tolerance for noise characteristics by return light.
For example, as a wavelength-tunable light source for use in Dense Wavelength Division Multiplexing (DWDM) optical communication, an integrated semiconductor laser element is disclosed in which a plurality of distributed feedback lasers are integrated of which laser oscillation wavelengths differ from each other (Japanese Laid-open Patent Publication No. 2005-317695).